I am really just beginning (the typical newbie looser way of saying with 'argh! Help me!).

I am going through Wheellock's Latin and have got to the section that talks about the first declension. There are examples such as

Nom: porta magna
Gen: portae magnae
etc

ok. I understand. There are different forms for the different cases.

Then I reached the 3rd chapter 'Second Declension"

And this has examples such as

Nom: amicus magnus
Gen: amici magni
etc

hmm.... now besides the fact that we are using one different word, and the the 'magnus' is here a slightly different concept (great instead of large) I fail to see the difference. I don't see an explanation in the book either. It only points out HOW the 2nd is different, but not WHY.
But as far as I can see, we have two components,
the noun and the adjective in both declensions...

okay, rule number 1: adjectives agree with the nouns they modify in case, number, and gender.

What this means:

You correctly recognized that there are different endings for different cases. To make things complicated, however, there is more than one set of endings. (These are what we mean when we talk about '1st declension, 2nd declension, etc)

So - some nouns will follow the pattern of -a in the nominative, -ae in the genitive, while others will have a patter of -us, -i or -um, -i. This is fairly arbitrary. There's no inherent reason in the meaning of the word why it should end in -a rather than -us.

1st declension nouns are (almost always) feminine in gender. 2nd declension nouns are masculine or neuter. Again, the gender is arbitrary, but the declension patterns are associated with certain grammatical genders.

Adjectives, however, have no inherent gender. They borrow the grammatical gender -- and hence their endings -- from the gender of the noun they are used with.

'Porta' is a feminine noun. Therefore, when you use the adjective magnus/a/um with it, it uses the form 'magna'. 'Amicus' is masculine, so the adjective form is 'magnus'.

Thank you! That did clear it up. So it is purely gender based!?
I have also started studying German, so I have gotten used to the arbitrary assignment of gender to Nouns. That I can begin to accept (if not enjoy ).
But anyway, now I think I understand.. I'm sure I am just too thick to understand it straight away.
Thanks again!
sean